In semiconductor integrated circuits, the formation of metal interconnect layers is important to the proper operation of such devices. Metal interconnect signal lines make contact to lower conductive layers of the integrated circuit through vias or through contact windows to "active" device regions of the semiconductor in an insulating layer. For best operation of the device, the metal used to form the interconnect layer should completely fill the via or contact window (hereinafter referred to as opening(s)).
Because of its physical properties, aluminum-alloys (e.g., aluminum-copper, aluminum-silicon, aluminum-copper-silicon) are especially suited for fabrication of metal interconnect lines in integrated circuits. However, the sputtering process used to apply aluminum-alloy thin film layers to an integrated circuit generally results in less than ideal filling of openings. Since the aluminum-alloy is deposited at an elevated temperature to obtain large grains for improved electromigration reliability, these large aluminum-alloy grains tend to form on the upper surface of the insulating layer. The grains that form at the edges of the opening tend to block the opening before the aluminum-alloy has a chance to completely fill it. This results in voids and uneven structures within the via.
This problem is especially acute as integrated circuit devices are fabricated using smaller geometries. The smaller openings used in these devices tend to have a larger aspect ratio (opening height to width ratio) than larger geometry devices, which exacerbates the aluminum-alloy filling problem.
The uneven thickness of the aluminum-alloy layer going into the opening has an adverse impact on device functionality. If the voids in the opening(s) are large enough, contact resistance can be significantly higher than desired. In addition, the thinner regions of the aluminum-alloy layer will be subject to the well known electromigration problem. This can cause eventual open circuits at the contacts and failure of the device.
To solve the problems associated with sputtering techniques, many approaches have been used to ensure good metal contact to lower interconnect levels. For example, one technique involves depositing the aluminum-alloy interconnect by sputtering in a physical vapor deposition tool ("PVD"), and then reflowing it in a separate reflow module at temperatures that range between 500.degree. C. to 575.degree. C. At these temperatures, the surface mobility and diffusion kinetics of the aluminum-alloy are enhanced, allowing it to deposit into the openings and fill them. However, at these high temperatures, the reflow module is very sensitive to impurities. As is well known, aluminum-alloy easily oxidizes and any presence of oxygen or moisture in the tool, particularly while processing in the reflow module, will negate the reflow process. In other words, the aluminum-alloy will not reflow and properly fill the opening if the aluminum-alloy oxidizes or moisture forms on it. Thus, the deposition, transfer and subsequent reflow are carried out in an ultra high vacuum environment, preferably in a multi-chamber cluster tool having very low partial pressures of water vapor and oxygen.
These environmental conditions require extensive pre-conditioning times, such as tool chamber pump-down and bakes, to bring the modules up to the required pristine operating conditions. Further, metal vacuum seals must be used instead of conventional "O"-ring seals to reduce module pump-down times. These metal seals must be used in the transfer chamber as well as in the deposition and reflow chambers. Consequently, these pre-conditioning steps increase the overall cost of the machine and the production time, which also increase the overall cost of the semiconductor devices.
Accordingly, what is needed in the art is a semiconductor device and a method of manufacture therefor such that the conductive layers connecting the openings are not subject to substantial oxidation during the manufacturing process. The semiconductor and method of the present invention addresses these needs.